In order to reduce the fuel consumption in motor vehicles, the internal combustion engine is paused or stopped when the motor vehicle is at a standstill and then restarted as soon as the motor vehicle is to drive on. This start-stop operation is used with a start optimization which allows the internal combustion engine to be started even before synchronization has occurred with the crankshaft. A prerequisite for the start optimization is that one knows the position of the engine after the internal combustion engine has stopped, so that, when a first tooth flank of the crankshaft sensor is detected, incrementation can be used to determine the current engine position up until synchronization. After synchronization, the start-up procedure is continued in the usual manner.
The different speed developments of an internal combustion engine with direct injection that is running up and which is operated with or without start optimization, are shown in FIG. 1. The curve with the broken line shows the speed development for an operation without start optimization, while the curve with the unbroken line represents the speed development for an operation with start optimization. The speed developments are entered on the same time axis, so that it is possible to make a direct chronological comparison of the two speed developments. One can see that at the point in time t=430 ms, the internal combustion engine with start optimization has already reached idling speed, whereas the internal combustion engine with the conventional start strategy fires for the first time at this point in time. The curve for the operation with start optimization shows marked fluctuations in speed while the engine is running up. These fluctuations occur because of the influence of the compression work that the cylinder has to perform, the influence of the friction and by an intermittent release of moments. A start-stop operation of this type is disclosed in DE 43 04 163 A1, the start optimization of which is shown in FIG. 2. One can see that the different cylinder charges of the initial combustion, for example in a four-cylinder internal combustion engine during the start optimization, i.e. before synchronization, depend on the stop position of the internal combustion engine. In the case of an internal combustion engine with more than four cylinders, it is also possible to have several initial combustions during the start optimization, which combustions influence the start behavior of the internal combustion engine. The dependence of the stop position occurs in particular for direct injection internal combustion engines. As only with direct injection after closing the inlet valve, fuel can still be inserted into the combustion chamber and thus combustion of a cylinder charge dependent on the stop position can be realized. In contrast, in the case of engines with inlet manifold fuel injection systems, the fuel injection is completed at the end of the inlet phase, so that there is always a maximum cylinder charge for the initial combustion for the start optimization of an engine with an inlet manifold fuel injection system.
As shown in FIG. 2, depending on the stop position of an internal combustion engine with four cylinders, it is possible to have one or several combustions during the start optimization. The stop positions and the positions discussed further here, are given respectively as crankshaft angles. Because of their cylinder charges these combustions form the varying speed development up to synchronization. Because of these varying initial conditions of the respective combustion during the start optimization, chemical energy is not reproducibly converted into kinetic energy. This is represented by way of example in FIG. 3, which shows and compares the speed developments and the rotary accelerations of the start up of the internal combustion engine starting from a stop position of 45° after the top dead centre and of 90° after the top dead centre. One can see that the rotary acceleration at approximately 4500 revs/min−1*s−1 at a stop position of 45° after the top dead centre is almost twice as great as the rotary acceleration of 2600 revs/min−1*s−1 at a stop position of 90° after the top dead centre. This varying dynamic during the start optimization results in a load moment which acts on the longitudinal axis of the vehicle, this is especially the case with vehicles with the internal combustion engine built in lengthwise because of the conservation of momentum. Subject to the stop position, this load moment varies in size and creates a corresponding tendency to roll in the motor vehicle. The driver of the motor vehicle will experience this tendency to roll as an “unrest” in the motor vehicle or as a disturbance in its “start experience”. Therefore, for driver comfort, it is necessary for the start behavior of an internal combustion engine to be reproducible regardless of its stop position in the start-stop operation.